Command Explanations

make
install-private-headers: This command is used to
install the Tcl library interface
headers used by other packages if they link to the Tcl library.

ln -v -sf tclsh8.6
/usr/bin/tclsh: This command is used to create a
compatibility symbolic link to the tclsh8.6 file as many packages
expect a file named tclsh.

sed -e ...: The
Tcl package expects that its
source tree is preserved so that packages depending on it for their
compilation can utilize it. These sed remove the references to the
build directory and replace them with saner system-wide locations.